Electrolytic process for producing oxide coatings on aluminum and aluminum alloys



States ELECTROI/YTIC PROCESS FOR PRODUCING X- IDE COATINGS 0N ALUMINUM AND ALUlWL NUM ALLQYS No Drawing. Application June 21, 1954 Serial No. 438,349

12 Claims. (Cl. 204-58) This invention relates to the production of hard wear and corrosion resistant aluminum oxide films on aluminum and aluminum alloys by electrolytic oxidation of the aluminum and the aluminum alloys.

As is well known in this art the anodic oxidation of aluminum to create oxide films in acid electrolytes, such as chromic acid, sulphuric acid, or oxalic acid, now generally employed, is limited to the production of thin films of oxide. Such films may not practicably be in excess of about .001 in thickness. The denseness of these films is also of moderate nature, being porous, especially when approaching in thickness the dimension of .001". While in some procedures coating up to .005" has been reported, the thickness which is recommended for commercial use is substantially under this figure. An additional limitation lies in the fact that the processes of the prior art are limited to certain only of the aluminum alloys. Thus, it has been reported that alloys contain ing high percentages of copper and/or silicon will not produce dense coatings of uniform thickness by such prior art processes. Further, in such prior art processes, contamination of the bath by copper and iron ions materially afiects the utility of the bath, and when such ions appear the current density must be substantially increased and soon reaches an impracticable value such as to result in corrosion, i.e., burning of the part being oxidized, and the bath must be discarded.

The process of my invention is capable of making thicker, denser and harder oxide coatings on aluminum and aluminum alloys than prior art processes known to me, and also produces thick, dense, hard oxide coatings on aluminum alloys which prior art processes are incapable of coating and is substantially insensitive to the presence of copper and iron ions in the bath. The current density at comparable voltage is not afiected in the process of this invention by the presence of copper or iron ions in the electrolyte bath. Improved bonding of the oxide coating to the metal is also obtained, and the coating is deposited more rapidly than heretofore in prior art processes.

I have discovered that by adjusting the electrolyte bath according to my invention I can operate at higher voltages than is used by the prior art and by suitably cooling the electrolyte bath I can produce the unique results of my invention.

I have found that by increasing the voltage and preferably also by drastically reducing the temperature of the electrolyte solution I may increase the thickness of the oxide coat and increase the rate of growth of the coating. I have also found that my results are facilitated by using a surface active compound in the electrolyte bath which is active in acid solutions.

I have also found that the results which are characteristic of my invention are obtained when I use as an additive to the acid bath an aqueous extract of lignite, brown coal, or peat, such as, for example, Georgia peat.

In preparing the electrolyte I may employ any of the acids usually employed in making up the electrolyte for electrolytic oxidation of aluminum such as sulphuric acid, chromic acid, or oxalic acid or mixtures thereof. The latter acids are considered equivalents for the anodic oxidation of aluminum and its alloys and are termed electrcanodizing acids herein. Other acids, in addition to sulphuric, oxalic and chromic acids, have been suggested by the prior art for the anodic oxidation of aluminum, and those skilled in the art will understand the nature and type of such acids contemplated herein.

I may use up to say about 70% H in the electrolyte, but I prefer to use dilute H 50 in an amount corresponding to a range of from about 1 part to 20 parts by volume of concentrated sulphuric acid dissolved in 100 parts by volume of water. For example, I may employ an amount of H SO in the electrolyte corresponding to from about 5 to 7% by volume of 66 Baum sulphuric acid per 100 parts by volume of water. I also employ in the bath from 1 to 6 parts, preferably about 3 to about 6 parts, by volume of the aqueous extract of brown coal, lignite or peat per 100 parts by volume of water in the anodizing acid solution. To this aqueous electrolyte I may and preferably do add an acid resistant wetting agent, usually in an amount of from 0.02% and higher, e.g. up to, 1% or more, by weight of the aqueous electrolyte solution. The upper limit of the wetting agent is determined, as a practical matter, by the formation of a high froth or suds, which is preferably avoided. A suitable wetting agent is the ether of nonyl phenol and polyethylene glycol.

In order to prevent freezing of the electrolyte at the low temperatures employed I may add an alcohol soluble in the electrolyte, such as methyl alcohol or any material which will lower the freezing point of the solution. I have found that about 7% by volume of methanol in the electrolyte is sufficient, although I may use more or less depending on the electrolyte composition and the temperatures employed.

It is understood that the above proportions of ingredients in the electrolyte may be varied if desired.

My extract additive can be obtained by extracting brown coal, lignite or peat obtained from various localities, with water. The extraction is particularly made more rapid and yield is increased by extraction at elevated temperature, preferably the atmospheric boiling point of the mixture or more elevated temperature, and most desirably by extraction at elevated temperature under pressure for a period sufiicient to produce an aqueous acid solution. I have found peat to be particularly suitable for this purpose and this is the preferred raw material. The peat used in practice of my invention may be derived from various locations in the United States, for example, from Georgia, Florida, California or Michigan.

In practice, the starting material, e.g., lignite or Georgia peat, is first mixed with water in a proportion of say one part by weight of the ground, flaked or fibrous lignite, peat or brown coal with, for example, six parts of water. These proportions may vary, however. This aqueous mixture is then fed to an autoclave when high extraction temperatures are desired for practical reasons and wherein the mixture is cooked at autogenous pressure and at temperature above its normal boiling point (i.e., the boiling point of the mixture at atmospheric pressure). In this respect I have found that satisfactory results according to the invention are obtained by cooking the brown or more, the time required to process the organic starting material being less as the temperature and pressure are increased.

While the above described high pressure and high temperature method for producing the extract is preferred, I can also obtain such extract by refluxing the aqueous mixture of brown coal, lignite or peat at atmospheric pressure over an extended period of say 24 hours. After extraction, the undissolved residue may be separated from the extract.

As a preferred additive I employ an aqueous extract of Georgia peat made according to the example described below.

Four parts by volume of Georgia peat ground and mixed is added to 30 parts of water and the resulting mixture is cooked at 20 lbs. pressure for 48 hours. The resulting material is cooled and allowed to settle. The liquid is decanted from the insoluble residue and forms the peat extract additive which is incorporated in the electrolyte according to the invention. This extract is in the form of an aqueous solution of organic acids of a complex nature, and is characterized by the following properties. The aqueous extract has a pH of from about 4 to about 6 and may contain as little as 2% of dissolved or dispersed solids, depending on the amount of dilution of the material. The extract upon evaporation to dryness leaves a dark brown, glossy residue which is amorphous and has a total nitrogen content of about 4.5 to for example, 4.8% as determined by the Kjeldahl method. The extracts solids are essentially water soluble and form a clear dark brown solution or dispersion. The aqueous extract is preferably kept refrigerated or a fungicidal material such as Dowicide A, believed to be essentially sodium o-phenyl phenate, is added to prevent mold growth.

I may use various types of acid resistant wetting agents, but I prefer to use non-ionic wetting agents, especially in the form of ethers produced by reaction of ethylene oxide on alkyl phenols or alcohols. These ethers should contain a sufficiently long chain of ethylene glycol units to render the ethers soluble in acid solution. The above type wetting agents are preferred since they are resistant to hydrolysis in acid solution. A preferred wetting agent is the ether of nonyl phenol and polyethylene glycol. Wetting agents of the foregoing type are commercially available.

The part to be coated is connected to the anode of an electrolytic cell by a wire and immersed in the electrolyte bath which is maintained preferably at a low temperature, usually less than about 50 F. The unique results of my process are improved as the temperature is lowered. I have used successfully temperatures ranging from 0 F. to 50 F. but the preferred temperature is in the range 0 F. to 35 F., e.g. 10 to F. However, the results are less marked at temperatures above about 35 F. Preferably, direct current is applied first at low voltage and then at rapidly increasing voltage until the desired thickness of coating is reached. If desired, however, an alternating current component may be added to the direct current. Coating thickness is dependent on time and voltage. As the coating becomes thicker, its electrical resistance requires higher voltages for penetration. The higher voltages cause local heating and the heated aqueous acid, e.g., H SO solution has greater dissolving power. By adding the lignite or brown coal or peat extract to the electrolyte I can use a substantially lower concentration of H 50 than is employed in the prior art and also preferably operate the bath at a substantially lower temperature than is employed in the prior art. The rapidincrease of voltage at the end of the coating period produces the proper thickness quickly With little softening or solution of the aluminum oxide coating; hence, harder and thicker coatings are obtained by my process than is'obtainableby the prior art.

In operating my process the voltage is gradually increased from a starting voltage determined by the: resist- 4 ance of the electrical circuit, to the starting voltage of the process which is in the range of from about 1 volt below the critical voltage to that determined by the resistance of the cell which may be at or below 40% of the critical voltage at which the coating starts to form. The initial or starting voltage depends on the nature of the aluminum surface or of the alloy when alloys are processed. The voltage is gradually increased from the starting voltage in the above range to the critical voltage, keeping the temperature low as stated above. A voltage is reached when anodic oxidation becomes observable in an amount to give a coating which is measurable by a micrometer, i.e., at least 0.0001 thick (the critical voltage). I note the current value when this occurs. At each increment of voltage or voltage step above this critical voltage the current rises as the voltage is increased. The voltage remains substantially constant for a period at each voltage step. During such period of constant voltage the current is at first constant for a period and then drops. This is caused, I believe, by the growth of the oxide coating which increases the electrode resistance. When the current decreases, I raise the voltage and therefore the current value, adjusting the voltage at each step to obtain approx mation of the current value attained when perceptible measurable oxide coating first started to form. The voltage may be increased to give a higher current value provided care is taken to avoid a current so high as to cause a conversion, known in this art as burning of the part. Such a burning occurs usually in localized portions of the anode and unless voltage is adjusted to drop the current value disintegration of the aluminum anode may occur. The voltage is increased in steps adjusting the voltage increment each time to approximately reestablish the above noted current value using the same precautions, and the current variation described above is repeated at each of these higher increments of voltage. The oxide coating increases in thickness until a point is reached where further increase in voltage acts to decrease the thickness of the coating.

Most desirably, operations according to my invention are carried out while maintaining the temperature of the electrolyte bath between about 0 F. and about 15 F. I have observed that if a light oxide coat is desired, it can be obtained about as easily while maintaining the electrolyte at 15 F. as at 0 F. However, if a heavy oxide coating, e.g., of .005 to .010 inch thickness, is desired, I prefer to maintain the temperature below about 15 F, e.g., at about 0 F. If the parts to be coated have restricted areas where good circulation is not obtained, it is generally preferred to operate at about 0 F. or at lower temperatures even where a light oxide coating is sought.

In my process I have foundv that aluminum and each of its alloys has a critical voltage at which oxide deposition occurs in measurable thickness. The magnitude of this critical voltage is dependent not only on the nature of the metal used, i.e., aluminum or its alloys, but on the electrolyte and other factors influencing the internal and external resistance of the cell. Below this critical volt age measurable growth, i.e., as available by micrometer measurement, to form a coat at least 0.0001 thick, is not observed.

The following list gives the critical voltage for aluminum and some of its alloys which I have found to be the practical lower limit for the start of the building up of measurable oxide coatings in my process. I have found as a practical matter that I must use at leastthe following voltages in order to obtain measurable oxide coatings within practical periods of time, for example, under 10 minutes. These voltages are for metal pieces of the same size treated in the same cell under the same conditions and show the relationship of these critical voltages for different aluminum alloys.

The voltages noted in the above list were measured by a standard'commercial D.C. voltmeter.

It is understood that the magnitude of the above potentials will change with the arrangement of the cell and its internal and external resistance; For each cell and each anode there is a critical voltage at which the deposit is made. This critical voltage is readily observable by visual inspection of the anode to determine the voltage at which deposit starts to form as described herein.

I have found that 'it is desirable to start the coating formation by adjusting the voltages from below the" critical voltage up to critical voltage where coating formation is initiated. Thus, for example, when treating aluminum, I adjust the voltage in the cell referred to above up to the 22 volts wherein it has been observed that coating starts to form. I have found that the coatting will form at lower voltage, i.e., at a voltage about 5 to below the critical voltage, but that in such case the time for forming the measurable coat is greatly prolonged, e.g., up to 1 to 2 hours. such. prolongation coat aluminum in the above cell at about 20 to 21 volts. However, if the voltage in the above cell is materially below 19 to 20 volts, I do not obtain any measurable coat on the aluminum in the above cell. Similar observations have been made with the aluminum alloys in the above cell. Thus, I believe that to produce useful coatings the voltage required is critical for practical operation to the extent that the voltage employed should be not more than about 5 to about 10% below the critical voltage value. That is, I prefer to start my operation by producing theinitial coat, i.e., the first measurable coat, at a voltage value substantially'equal to the critical value and-not less than about 90% of the critical value.

In practical operation if the process is not initiatedat or below the critical voltage within the limits described above, but at a higher voltage, for example, about l05-110% of the critical voltage, burning of the aluminum or aluminum alloy part results; that is, an actual destruction (pitting and dissolution) of the part being coatedresults which, if continued, could cause the part to substantially dissolve. voltage for initiation of oxide coat formation preferably should not depart from the critical voltage in amount more than or less than about 10% of the critical voltage. I prefer to initiate the operation at at least one volt below the above critical voltages; thus, for the aluminum and its alloys stated in the above table, I prefer to initiate the oxidation of such part at least one volt below the critical value given in the table. But for some objects, particularly of small cross-sectional area such as thin aluminum wire or thin aluminum sheets, I desire to initiate the process at at least about 5 to 10 volts below the critical voltage for aluminum.

It is noted that when a pure aluminum wire or an alloy wire of aluminum is used to connect the part to be coated with the cell bus bars, and the wire has a critical voltage less than that of the part to be coated, it is necessary to initiate the process at the critical voltage of the wire; in other words, the wire must first be anodized. If a connector wire is used which is of the same composition as the part to be anodized, or of any other composition having the'same ora higher critical voltage than said part,

Thus, I may 'by Consequently, the practical operations can-commence at a voltage closely approaching or atthe critical voltage of the part to be oxidized. The operation can be also initiated at a voltage just below or at about the critical voltage of the part to be coated by using an anode connecting wire which is not attacked by the electrolyte, for example, a wire such as titanium wire or noble metal wire, or aluminum wire or other metal which is attacked by the electrolyte plated with a noble metal such as silver or any other material to protect it against the electrolyte.

After initiating the oxidation below the critical voltage; operationis continued at any given voltage after each successive increment of voltage above the critical for say 2 or 3 minute intervals. After each time intervalthe voltage is increased by an addition of an increment of voltage. Such increments of voltage may be, when close to the critical voltage, about 1 to 3 volts, but such increments may be of greater value as the oxide formation As agood operating technique the amperage duringeach voltage step should be permitted to drop materially while maintaining the voltage substantially constant, to, for example, about 30% to 50% of the current value initially attained at each such voltage step after the incremental increase in voltage.

At each voltage increment after oxide formation has started the amperage should rise and start to fall after but a small interval of time within about the first 30 second after the voltage increment has been applied. If this phenomenon does not occur, the usual consequence is that the amperage will steadily increase, usually rapidly, and burningresults. This is indicative that the voltage increment was too great, unless some mechanical or electrical failure is the cause of this rise. Thus, the voltage increment should in each case be less thanthat which permits such excessive current flow. Asa practicel guide, it is desirable to limit thevoltage increment so as to establish a current value not more than about to of'the amperage obtained when the previous voltage increment was applied, for example, not greater than 110-130% of the maximum amperage obtained at the critical voltage. Preferably, also, the voltage increment should be less than that which gives the burning phenomenon previously described.

Up to the critical voltage the magnitude of the voltage increments are not critical, nor is the amperage control critical as affecting the part to be coated with the oxide. As the critical voltage is approached the voltage should be changed about one or two volts at a time.

voltage and the current value is observed during the initiation of oxide formation at such critical voltage. As oxide formation occurs the current drops. When it has dropped to the degree specified above, the voltage is increased to reestablish approximately the value of the current observed at the initiation of oxide formation. I have observed as practical guide that if the amperage value is dropped to approximately 30% to 50% of the initial value, that a voltage increment of about 1 or 2 volts is generally sufficient during the early stages of oxide formation to reestablish the aforementioned current value which will give good oxide coating without burning. Subsequent voltage'changes are adjusted to reestablish-at the initiation of each voltage step increase the amperage found safe, i.e., inorder 'to obtain deposition withoutdestruction' of the oxide coat. The latter amperage corrcsponds approximately to the amperage at the critical voltage. During the latter stages of oxide formation, voltage increments of 'say 3 to 5 volts are usually required to approximately reestablish such current value.

The voltage may be increased according to the invention to and above 100 volts, and as high as about 130 volts, to wit', to the vo'ltageat which the coating no longer increases in thickness. During the operation, current density is generally maintained at less than 20 amps/sq; ft.,

Thus, the: oxide coating is initiated at, for example, the critical often dropping below amps/sq. ft. Up to about 100 volts the growth of oxide on a particular aluminum alloy is uniform and reproducible. Above 100 volts I have found that the condition of the alloy, i.e., its porosity, grain size and density, affect the growth materially so that in many cases, similar alloys will react non-uniformly at these high voltages. Thus, while most aluminum alloys will produce a coating say of .006" thickness at 100 volts in my process, the coatings of some will increase to .010" at 130 volts, while other exactly similar alloys differently treated or handled differently in fabrication will not so improve in growth by an increase in voltage.

from 100 to 130 volts.

Examples of hard commercial coating thicknesses ob- Although thicker coatings than those listed above have been obtained, the above are excellent hard coatings which may be obtained and are reproducible in operation, and are not obtainable by prior art processes known to applicant.

All of the above coatings are as uniform and as dense as a .001" coating of the same alloy produced by stopping the process at such point. In contrast to this, alloys containing more than about 3% copper could not be coated even with a light oxide coat by conventional prior art processes.

Oxide coatings on various other alloys in addition to those noted above and including the following may be obtained by my process:

Aluminum alloy designation Approximate composition 04% A1, 4.4% Cu, 0.8% Si.

I1. 85% Al, 4% Cu and 11% Si. 89% Al, 11% Mg.

A 4" x 4" x A" thick test panel of 248T aluminum alloy containing 4.5% copper was connected to the anode of an electrolyte bath contained in a stainless steel tank which formed the cathode. The electrolyte was prepared by adding about 3% by volume of the aqueous extract of Georgia peat produced as described above to a water solution of sulphuric acid formed by the addi tion to water of about 7% by volume of 66 Baum sul phuric acid. About .02% by weight of the ether of nonyl alcohol and polyethylene glycol was added as wetting agent and about 7% by volume of methanol also added.

In the table below is given the time during which each 8 applied voltage was maintained constant, the voltage to which the bath was raised in steps and the coat thickness obtained at each step.

Voltage Time Coat thickness (inches) Increase slowly (take 1 min.) After 02 min.

to 15 volts. Increase to 24 volts After 04 min Increase to 26 volts After 06 min- Increase to 27 volts After 08 min Increase to 28 vo ts" After 09 min Increase to 29 volts After 10 min Increase to 30 volts" After 12 min- Increase to 31 volts- After 15 min- Coating starts to form. Increase to 32 volts" After 18 min- Increase to 33 volts After 21 min- Increase to 34 volts After 23 min Increase to 35 volts After 25 min Approx. .0002. Increase to 36 volts After 27 min Increase to 37 volts. After 29 min .0004. Increase to 38 volts After 31 min Increase to 39 volts After 33 min- Increase to 40 volts After 35 min. .001. Increase to 42 volts After 37 min Increase to 44 volts" After 39 min Increase to 46 volts After 41 min Increase to 48 volts After 43 min. Increase to 50 volts After 45 min .0015. Increase to 53 volts. After 47 min Increase to 56 volts After 49 min. Increase to 58 volts After 51 min. Increase to 60 volts After 53 min .002. Increase to 64 volts. After 55 min Increase to 68 volts After 57 min Increase to 72 volts After 59 min. .003. Increase to 76 volts After 61 min- Increase to volts" After 63 min .004. Increase to 34 volts After 65 min Increase to volts After 67 min .005. Increase to 05 volts After 69 min Increase to volts After 71 min. .007.

The above results have been found to be obtainable also with other copper-containing aluminum, wrought of forged alloys containing copper above about 3%.

It will be observed from Example 1 that at 90 volts the thickness of oxide coating reached .005", and at 100 volts the coat thickness was .007". It was observed that even at the higher coat thickness of .005" to .007", the oxide coating was dense, hard and uniform.

EXAMPLE 2 An aluminum alloy designated as No. 356 containing 7% silicon in the form of a test panel of the size in Example 1, was mold cast and machined to a 32 micro finish and treated in the electrolyte of Example 1. The following table gives the results obtained:

Voltage Time Coat thickness (inches) Start generator 00 Increase to 15 volts After 02 min Increase to 26 volts After 04 min Coating commenced. Increase to 27 volts A After 06 min- Increase to 28 volts After 08 min mcrease to 29 volts After 10 min Increase to 30 volts After 12 min- Increase to 32 volts After 14 min. Increase to 34 volts After 15 min Increase to 36 volts After 18 min .001. Increase to 38 volts After 20 min Increase to 40 volts After 22 min. Increase to 44 volts After 24 min- Increase to 48 volts After 26 min Increase to 52 volts After 28 min Increase to 56 volts After 30 min .002. Increase to 02 volts After 32 min- Increase to 68 volts After 34 min .003. Increase to 74 volts After 30 min. Increase to 30 volts After 33 min .004. Increase to 86 volts After 40 min Increase to 92 volts After 42 min .005. Increase to 100 volt After 44 mm Increase to volts. After 40 mm .006. Increase to volts. After 48 min Increase to 112 volts. After 50 mm Increase to 114 volts. After 52 min Increase to 116 volts After 54 min Increase to 118 volts After 56 min Increase to 120 volts After 58 min .008. Increase to 122 volts After 60 min Increase to 124 volts After 62 min Increase to 126 volts After 64 min Increase to 128 volts After 66 min Increase to 130 volts After 68 min. .010.

It is noted from the above table that a coat thickness of .005 was obtained at 92 volts, and a coating of .010" obtained at 130 volts.

EXAMPLE 3 The following tests were made under exactly the same conditions in all cases except that the electrolyte solution was different in each test.

The aluminum alloy to be coated in each case was 24ST-4 (unclad) having 45% copper. The test pieces were 6" long, 4" wide, and .081" thick.

Aluminum wire was wrapped around the center of each panel and used as the anode current carrier from the copper bus bar to the panel immersed in the electrolyte. 7

Temperature of the electrolyte was 12 F.

Agitation was obtained by air nnderpressure bubbling through the electrolyte.

Current was supplied by a DC. motor enerator set.

The tank carrying the electrolyte was made of stainless steel and served as the cathode.

Electrolyte make-up of Test #1:

7% by volume of 66 Baum water white H 80 V 93% water by volume Electrolyte make-up of Test #2:

7% by volume of 66 Bau'm water White, H 80 93% water by volume .02% by weight of aqueous acid electrolyte of the acid resistant Wetting agent employed in Example 1 Electrolyte make-up of Test #3:

7% by volume of 66 Baum water white H 80 90% Water by volume 3% by volume of Georgia peat extract prepared in the manner described above Electrolyte make-up of Test #4:

7% by volume of 66 Baum water white H 80 90% water by volume 3% by volume of Georgia peat extract obtained the manner described above I .02% by Weight of the aqueous electrolyte of the acid resistant wetting agent employed in Exam (Panel removed because amperage increased too rapidly. Panel burned in one corner. Balance of panel had clear glazed appearance. Measured physical oxide growth of .0001.)

Test N0. 2

Time (minutes) Volts Amps Start After 03 raise to 25 15 After 06 raise to 27 14-10 After 09 raise to 29 15-10 After 12 raise to 30 16-14 After 15 raise to 31 18-17 After 18 raise to; 32 20-20 After 21 raise to. 33 22-22 After 24 raise to 34 (Panel removed because amperage increased too rapidly. Part burned in one corner. Balance of panel had glazed appearaneeyvithafew golden spots over the surface. Measured physical oxide growth of .00025.)

Test Na. 3

Thickness Time (minutes) Volts Amps. of oxide coating (inches) Start 00"; After 03 raise to 25 After 06 raise to 27 After 09 raise to 29 After 12 raise to 31 After 15 raise to 32 After 18 raise to 33 After 21 raise to 34 After 24 raise to.... 36 After 27 raise to 38 After 30 raise to 40 After 33 raise to 42 After 36 raise to. 44 After 39 raise to 46 After 42 raise to 49 After 45 raise to 53 After 48 raise to 57 After 51 raise to '61 After 54 raise to 65 After 57 raise to 70 After 60 raise to 75 After 63 raise to After 66 raise to After 69 raise to After 72 raise to After 75 raise to Test N0. 4

Thickness Time (minutes) Volts Amps. of oxide coating (inches) After 57 raise to After '60 raise to After 68 raise to After 66 raise to After 69'raisetto" After 72 raise to Tests 3 and 4 were carried out in a manner similar to Tests 1 and 2. The voltage was increased in steps to the values indicated and maintained substantially constant during the time interval indicated. The amperage rose at the start of each voltage step and dropped through the range indicated in the above tables while the voltage Was being maintained constant during each such step.

Example 3 clearly shows the advantages of the use of my extract additive in the electrolytic bath when used in my process. It is seen that when my additive was not incorporated in the aqueous sulphuric acid solutions used in Tests No. l and No. 2, the amperage increased so rapidly even at low voltages of 32 to 34 volts that the panels burned, Whereas in Tests No. 3 andNo. 4, involving the use of my additive in the sulphuric acid solution according to the invention, amperage remained relatively low even up 100 volts, at which voltage a thick, hard, dense oxide coating Varying from .0055" to .0062" was ob tained. t

The surface coatings produced by my process have the following unique characteristics: 7

Analysis of the hardened surface shows it to range from amorphous aluminum oxide to very close to corundurn. This range of analysis is dependent upo the aluminum-containing material being processed, i.e., upon the purity ofthe ammmumorupen the amount and-na ture of the alloying agents of the particular aluminum alloy used. For this reason, hardness tests (Moh Scale) vary from 5.5 to 8.5.

Surfaces processed by my method on threads, splines, or irregularities are uniform in thickness on crests and valleys, and the parts will coat uniformly regardless of their position in the tank, and with a large number of parts being anodized in the tank.

Various hard coating surfaces were tested on the Taber Abraser device using C517 wheels of 1000 gram weight at 70 r.p.m. The results are noted below:

Coating failed at following number of revolutions Conventional chromic acid anodize on 24ST aluminum alloy (45% Cu) r Conventional hard chrome electro plate coating on 24ST alloy .0015 thick 1,000

Invention coating on 24ST alloy .0015" thick 33,500 Invention coating on 24ST alloy .0025" thick 69,800 Invention coating on 24ST alloy .003" thick 93,000 (Failure is construed to be any visual destruction of the coating.)

The panels produced according to Example 1 having .001", .002", .003" and .005 thickness of coating were subjected to 7,000 hours of salt spray treatment and to 1,000 hours partial submersion in exhaust gas condensation, after heating to 500 F. for 8 hours, without evidence of corrosion. For corrosion protection a coating Over .0005" in thickness is considered adequate for either salt atmospheres or exhaust gas vapors.

Hard surfaced aluminum oxide coatings produced according to the instant process were not afiected by low temperatures. Panels with .003 thickness cooled to 58 F. were held with cooled pliers and flexed rapidly 20 times, hammered with a hard rubber mallet 20 times, and struck against concrete. Fine lines appeared perpendicular to the direction of bend, but no flaking or peeling occurred. Other panels containing oxide coatings according to the invention were cooled to 58 F. and dropped in tap water, and showed no checking. Invention coated panels at -58 F. were transferred to a 450 F. oven and no change in the coating was observed after 6 hours at 500 F.; 3 hours longer at 600 F.; and one hour longer at 800 F. The oxide coated panel at 800 F. quenched in tap water showed checking, but no flaking or peeling. Invention panels with thinner oxide coats than .003 were less affected by bending and quenching.

The hardened oxide coating of the invention penetrates the parent metal and also builds up on the metal. The ratio of the penetration to build up is about 56% penetration and 44% build-up on coatings up to .003" thick.

This ratio varies somewhat with the alloys and with the form of the metal, i.e. whether it be wrought, forged, permanent mold cast, die cast or sand cast. I believe this variance is due to the density or porosity of the metal, or to the amount of alloying agents such as copper, chromium, silicon, and nickel used in the aluminum alloy. However, this ratio applies generally to the Wrought alloys and specifically to 24ST clad material polished. It also applies to certain of the cast alloys and specifically to the permanent mold cast alloy of Example 2 above. Hence, improved bonding of the oxide to the parent metal is obtained by my process, and coatings with uniformly good bonding at all thicknesses including maximum can be produced.

According to my process as described above, thicker oxide coatings on aluminum and its alloys are obtained and I am able to apply any desired coating thickness more quickly. This is advantageous, as the coating of aluminum oxide grows on and penetrates into the surface. I thus obtain over double the coating thickness of the prior art because of several factors: First, I am able to use high voltages without burning out spots of aluminum oxide. A given thickness of coating is obtained more quickly and at lower temperatures and with a lower concentration of H All of these factors decrease the attack on the aluminum oxide coating. The improvements obtained by the use of higher voltage, lower temperature and lower H 80 concentration in turn are made possible by using the additive in the electrolyte according to the described procedure.

Instead of using the aqueous extract of lignite, brown coal or peat as an additive in the electrolyte employed in the invention process, other additives may be used which permit the technique and procedure of the invention as described above to be applied to the electrolyte for obtaining the results of the invention. Thus, such additives should function to permit substantially increasing the voltage in the manner described above without substantial increase in current over that applied at the critical voltage, while operating at reduced electrolyte temperatures, to obtain oxide coatings which are thick, hard and dense, according to the invention. Since my additives permit such increase in voltage and consequent increase in coating thickness in a shorter period of time than in the absence of such additives, as shown in the examples above, the additives previously described when used in my process may be termed oxide coating accelerators. By this term is meant that the additives function to increase rate of growth of oxide coating by permitting increased voltages without producing burning, when employed in conjunction with the reduced temperature operation and the stepwise voltage technique described above. Other suitable additives for my invention include, for example, Z-aminoethyl sulphuric acid, taurine, and alkyl taurines, as for example, N-methyl taurine and N- cyclo-hexyl taurine, and sulfamic acid. The use of the latter specific compounds as additives in electrolytes for the electrolytic oxidation of aluminum and its alloys is described and claimed in the applications of Robert Ernst, Serial Nos. 457,314, 457,315 and 457,316, all filed September 20, 1954, and now Patents Number 2,855,350, 2,855,351 and 2,855,352, respectively.

While I have described a particular embodiment of my invention for the purpose of illustration, it should be understood that various modifications and adaptations thereof may be made within the spirit of the invention as set forth in the appended claims.

I claim:

1. A process for coating aluminum and aluminum alloy metal articles with a hard and tough coating of oxide of aluminum, which comprises passing an electric current through an electrolytic cell containing an electrolyte maintained at a reduced temperature with said article forming the anode, said electrolyte comprising a water solution of an electro-anodizing acid, establishing the voltage applied across said cell at a value in a range operative to produce a measurable coating of oxide of aluminum on the article Without burning, establishing said applied voltage at successively higher values by voltage increments, each said voltage increment being chosen to reestablish approximately the current initially obtained at the first-mentioned voltage, and maintaining the voltage at each said established value until said current decreases by a material percentage.

2. A process as set forth in claim 1, wherein said electroylte further includes an oxide coating accelerator material.

3. A process as set forth in claim 2, wherein said electrolyte is maintained at a temperature less than about 50 F.

4. A process as set forth in claim 2, wherein said accelerator material is an aqueous extract of a substance of the group consisting of brown coal, lignite, and peat, said extract obtained by "cooking a mixture of said substance and water at an elevated temperature, and the first 13 mentioned voltage is approximately the critical voltage of said article.

5. A process as set forth in claim 2, wherein the voltage increments during later stages of the process are substantially greater than during earlier stages thereof.

6. A process as set forth in claim 2, wherein the magnitude of the voltage increments is at least about one volt.

7. A process as set forth in claim 4, wherein said electrolyte is maintained at a temperature less than about 50 F., and said substance is peat.

8. A process as set forth in claim 5, wherein the magnitude of the voltage increments in said later stages is at least in the range of about three to five volts, and the magnitude of the voltage increments in said earlier stages is at least in the range of about one to three volts.

9. A process as set forth in claim 7, wherein the temperature of said electrolyte is maintained between about F. and about 35 F., said electro-anodizing acid is sulfuric acid, the mixture of peat and water is subjected to a temperature above the normal boiling point of said mixture under autogenous pressure over an extended period, and the extract has a pH of from about 4 to about 6.

10. A process as set forth in claim 7, and continuing the process until the voltage is at least 100 volts, the electro-anodizing acid being sulfuric acid.

11. A process as set forth in claim 11, wherein said electrolyte is maintained at a temperature between about 0 F. and 50 F., said electro-anodizing acid is an aqueous solution of sulfuric acid, said extract is present in said electrolyte in amount of from about 1 to 6 parts by volume per 100 parts of water in the aqueous sulfuric acid solution, said cooking is conducted at a temperature above the boiling point of the mixture and under autogenous pressure over an extended period, the Icurrent reestablished at each voltage increment is not more than about 130% of that obtained at the first-mentioned volt-' age, and said large percentage of current decrease is about 30 to 50%.

12. A process as set forth in claim 9, wherein the temperature of the electrolyte is maintained between about 0 F. and about 15 F.

References Cited in the file of this patent UN lTED STATES PATENTS 

1. A PROCESS FOR COATING ALUMINUM AND ALUMINUM ALLOY METAL ARTICLES WITH A HARD AND TOUGH COATING OF OXIDE OF ALUMINUM, WHICH COMPRISES PASSING AN ELECTRIC CURRENT THROUGH AN ELECTROYLTIC CELL CONTAINING AN ELECTROLYTE. MAINTAINED AT A REDUCED TEMPERATURE WITH SAID ARTICLE FORMING THE ANODE, SAID ELECTROLYTE COMPRISING A WATER SOLUTION OF AN ELECTRO-ANODIZING ACID, ESTABLISHING THE VOLTAGE APPLIED ACROSS SAID CELL AT A VALUE IN A RANGE OPERATVIE TO PRODUCE A MEASURABLE COATING OF OXIDE OF ALUMINUM ON THE ARTICLE WITHOUT BURNING, ESTABLISHING SAID APPLIED VOLTAGE AT SUCCESSIVELY HIGHER VALUES BY VOLTAGE INCREMENTS, EACH SAID VOLTAGE INCREMENT BEING CHOSEN TO REESTABLISH APPROXIMATELY THE CURRENT INITIALLY OBTAINED AT THE FIRST-MENTIONED VOLTAGE AND MAINTAINING THE VOLTAGE AT EACH SAID ESTABLISHED VALUE UNTIL SAID CURRENT DECREASES BY A MATERIAL PERCENTAGE. 